Generally, in a fluorescent lamp, ultraviolet rays caused by arc discharge are irradiated on a fluorescent material coated on the inner wall of a glass tube, and visible light caused by the fluorescent powder is used.
Also, glass used in the fluorescent lamp can be roughly divided into two parts, i.e., a cylindrical bulb part and a stem part. The bulb part is normally in the form of a straight tube or a circular tube obtained by heat-forming the straight tube. In recent years, for the purpose of enhancing the lamp efficiency or downsizing the lamp, fluorescent lamps having a complicated shape such as a U tube and a twin tube having two U tubes connected to each other have been developed.
At first, the glass tube to be used as the envelope for these fluorescent lamps having a circular or complicated shape had been formed by lead glass containing Pbo in an amount as relatively large as about 20 to 30% to facilitate working. At present, however, the lead glass is replaced by soda-lime glass to avoid the toxicity problem of PbO. The soda-lime glass for this purpose contains BaO as a component for lowering its viscosity so as to have workability close to the PbO-containing glass or contains Sb2O3 so as to maintain a high luminance required for fluorescent lamps.
However, the soda-lime glass containing BaO or Sb2O3 is disadvantageous in that since it has a high ultraviolet transparency, ultraviolet rays irradiated from the light source during lighting pass through the glass, giving adverse effects on human being or causing discoloration or deterioration of a resin constituting the lamp fixture.
In order to lower the ultraviolet transparency of glass for reducing the leakage of ultraviolet rays, CeO2 can be incorporated in the glass. However, when the glass containing CeO2, solarization, i.e., coloring of the glass by ultraviolet rays, is caused and hence the transmittance of the glass is lowered so that the luminance of the lamp is deteriorated.